1. Field of the Invention
The present invention relates to a signal transmission device having ALC (.Automatic transmission Level Control) function and also to transmission terminal equipment having the signal transmission device. More particularly, the present invention relates to a signal transmission device having ALC function effective in a system where the number of carriers changes in bursts by, for example, DAMA (demand-assignment activation), voice activation, etc. in a radio satellite communication system. The present invention also relates to multi-carrier transmission terminal equipment having the signal transmission device.
2. Related Prior Art
In an SCPC (Single Channel Per Carrier) system used in radio satellite communication or subscriber radio communication, for example, in the case of a DAMA system, a carrier is sent out after a communication channel (i.e., carrier frequency) has been assigned from the master station in response to a request from a slave station. In the case of a voice activation (or carrier on/off) system, a carrier is sent out as long as a speech input is present.
In such a system, since the number of carriers sent out changes with time, the total power, which is the target of ALC, also changes with time. Accordingly, the system is required to have a dynamic; range sufficiently wide to cover the range in which the number of carriers may change for detection of the output level.
FIG. 1 shows one example of the arrangement of a conventional signal transmission device having ALC. The illustrated arrangement is disclosed in the invention of U.S. patent application Ser. No. 851,614, made by the inventors of this application.
Referring to FIG. 1, a transmission circuit 1 includes a frequency converter for converting an intermediate-frequency (IF) signal into a radio-frequency (RF) signal, a high-power amplifier, etc., The transmission circuit 1 modulates a carrier from a local oscillator in accordance with an intermediate-frequency (IF) signal and amplifies the modulated carrier to generate a transmission output. The transmission output is sent to an antenna through a hybrid circuit 7.
A detection circuit 2 detects the level of the transmission output distributed from the hybrid circuit 7 and outputs a detection signal VD of direct-current (DC) voltage corresponding to the detected level.
A reference voltage generating circuit 3 generates a reference voltage VR the size of which changes according to information showing the number of carriers. The information showing the number of carriers is obtained from a control device (described later). Alternatively, the information may be produced from the signal transmission device itself as is disclosed in the aforementioned U.S. patent application Ser. No. 851,614.
A control circuit 4 detects a difference voltage between the reference voltage VR and the detection voltage VD and inputs a control signal VC to a variable attenuator circuit 5 (described later) so that the difference voltage converges to zero.
The variable attenuator circuit 5 controls the gain of the input signal on the basis of the control signal VC corresponding to the difference voltage, sent from the control circuit 4, to generate a modulating signal for the transmission circuit 1.
Thus, in the arrangement shown in FIG. 1, the detection circuit 2 generates a detection voltage VD which changes with the number of carriers. In the meantime, the reference voltage generating circuit 3 changes the setting of the reference voltage VR, which is the control target, according to the information showing the number of carriers. The control circuit 4 compares the detection voltage VD with the reference voltage VR and controls the gain of the variable attenuator circuit 5 on the basis of the result of the comparison, thereby maintaining the transmission output per carrier at a proper value.
When the number of carriers changes in the above-described signal transmission device, the dynamic range in the detection circuit 2 becomes a matter of great concern. What the detection circuit 2 observes is the total output signal level. Therefore, the proper transmission level for the transmission of 1 carrier differs from that for the transmission of 6 carriers by EQU 10 log(6/1)=7.78 dB
That is, for the same power per carrier, the total power for 6 carrier's is higher than that for 1 carrier by 7.78 dB.
In general, the detection circuit 2 comprises a detector using a diode. However, the range of diode's square-law characteristic usable for measurement of power is limited. Accordingly, there are upper and lower limits on the detectable level range, which determines the dynamic range of the detection circuit 2.
In the present state of art, the dynamic range is 15 dB at the most.
On the other hand, it is not rare that the range of level variation necessary to compensate for by ALC in the system exceeds 10 dB when all level variation factors to be compensated are taken into consideration, i.e., variation of the gain of the transmission circuit 1 with temperature, variation of the cable length, variation of each component, and so forth.
FIG. 2 shows one example of distribution of the dynamic range of the detection circuit 2. If the range of level variation factors to be compensated by ALC is assumed to be 10 dB, the range of level variation by 6 carriers is 7.78 dB. Accordingly, if the dynamic range of the detection circuit 2 in the system is 15 dB, the variation range lacks about 3 dB.
In such a case, level compensation may be made for each component in order to compress the level variation factors to be compensated. Alternatively, ALC may be divided into two stages to make compensation at each stage.
FIG. 3 shows one example of a signal transmission device having ALC composed of two stages. In the figure, a pair of transmission circuits 1a and 1b, a pair of detection circuits 2a and 2b, a pair of reference voltage generating circuits 3a and 3b, a pair of control circuits 4a and 4b, and a pair of variable attenuator circuits 5a and 5b respectively correspond to the transmission circuit 1, the detection circuit 2, the reference voltage generating circuit 3, the control circuit 4, and the variable attenuator circuit 5, which are shown in FIG. 1.
In the arrangement shown in FIG. 3, the transmission circuit in the signal transmission device is divided into two stages, i.e., the transmission circuits 1a and 1b, to effect amplification and other required operation, and the detection circuits 2a and 2b are provided to correspond to the two transmission circuits, respectively, for detection of output levels. Subsequently, the detected output levels are compared with respective reference voltages supplied from the reference voltage generating circuits 3a and 3b in the control circuits 4a and 4b to obtain difference voltages, and the gain is controlled in the variable attenuator circuits 5a and 5b on the basis of the difference voltages, thereby effecting ALC.
Accordingly, the signal transmission device shown in FIG. 3 has a dynamic range equal to the sum of the respective dynamic ranges of the detection circuits 2a and 2b and is therefore capable of realizing the desired ALC.
However, either of the conventional methods, in which level compensation is made for each component, or the signal transmission device is composed of two stages, suffers from the problem that the hardware scale increases and the system cost rises unavoidably.